Analysis of adhesion mechanism and influencing factors of waterborne coatings

Good adhesion of the coating to the surface to be coated is the premise
for the coating film to exert its protective role.
The adhesion of the coating is one of the most important indicators of
the coating during application.

01.
Mechanism of adhesion

The importance of adhesion has long been known, but until now, the mechanism of adhesion in coatings has not been fully unders coatingol.
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tood.
Most explanations of adhesion are based on experience and hypotheses, but we can also use it as a means to make dialectical arguments
.

Mechanical adhesion

The mechanism of this adhesion is mainly for coating on substrates with uneven interfaces containing holes, holes or cracks
.
When the paint liquid is applied to the surface of the substrate, the paint can wet and penetrate into these gaps, so that after the paint film is cured, the paint film that enters the gap can act like a nail, thereby grasping the substrate
.

The mechanism of mechanical adhesion has certain manifestations
in such as wood, sandblasting, mechanical grinding, etc.

Covalent bond theory

The theory of covalent bonding refers to the formation of certain covalent bonds after the coating film comes into contact with the substrate, and this type of link force is
the strongest and most durable.
The premise of formation requires that there are chemical groups that fit each other in the coating film and substrate, and these groups can form chemical bonds
when they are close together.

For example, the adhesion of some organosilanes to glass, metals and organic ores is enhanced; Another example is that some substrates have hydroxyl groups, carboxyl groups and the curing agent in the coating.

Van der Waals force theory

Van der Waals forces are the most common force
that provides conventional adhesion.
When the coating film is in close contact with the substrate, the molecules are close to each other and create attraction
.

This attraction is a short-range interaction, generally with a strong effect
at 0.
3-0.
5nm.
In addition, the larger the relative molecular mass of the molecule, the greater the deformity, and the easier it is to be polarized, the stronger the force
.

Electrostatic theory

There is a certain residual charge dispersion on both the surface of the coating and the substrate, and when the paint and the substrate are in close contact, these charges attract each other to promote the adhesion
of the paint film.

Hydrogen bonding theory

When hydrogen atoms form covalent bonds with atoms with large electronegativity (such as O, N, F, etc.
), the electrons are biased towards atoms with high electronegativity, and hydrogen atoms are positively charged
.

Therefore, when the coating contains groups similar to -OH and -COOH, and when the substrate contains negative electric groups, a hydrogen bonding effect may be formed, thereby improving the adhesion
of the coating film.

This type of hydrophilic group may affect the water resistance of the coating film
.

Diffusion theory

When the coating film is applied to the substrate, the two affinity macromolecules may form a staggering through diffusion, which improves adhesion
.
This force can only occur between the coating and the substrate that are compatible with each other, so it mostly acts on the adhesion between layers, especially between the two coatings of the unified system
.

The above is a common explanation
of the mechanism of adhesion formation.
In fact, in the coating of coatings, the mechanism of adhesion is diverse, often containing a variety of forces
.

02.
Influencing factors of adhesion

From the above analysis of various mechanisms, it can be seen that regardless of the form of force to promote adhesion, the close adhesion of the coating to the substrate is the primary condition
.
By tightly bonding and reusing a special attachment structure of resin or additives, a strong and tough paint film
adhering to the substrate can be obtained.

It can be seen that different aspects such as the performance of the resin itself, the formula structure, and the coating will have a certain degree of impact
on the adhesion.

Characteristics of water-based resins

When it comes to performance influencing factors, the first thing to talk about is the performance effect of the waterborne resin itself, and adhesion is no exception
.
And the performance characteristics of waterborne resin itself are the most important influencing factors
of adhesion.

The wetting fluidity, particle size, special groups, surface energy and cohesion after forming a dry film of waterborne resin will greatly affect the adhesion of
the coating film.

Take SV-6145 with excellent adhesion as an example:

(1) The resin has good wetting fluidity, and through good wetting, flow and spreading, the paint film can be in closer contact
with the substrate.

(2) The resin particle size is small, which prevents the steric hindrance effect formed by excessive latex particles, affecting the effective contact between the paint film and the substrate, simply put, the film formation is more dense
.

(3) It also contains a special anchor group and has a low cohesion
.
Through the linkage effect of special groups, excellent adhesion is formed, which is enough to offset the effect
of its low cohesion on the coating film.
In general, softer resins have more advantages in adhesion, because they are more able to adapt to different degrees of deformation of the substrate as the environment changes
.

SV-6145 still ensures good adhesion when it has excellent hardness
.

Substrate treatment

Substrate treatment, such as grinding, sandblasting, etc.
, can effectively increase the roughness
of the substrate.
The increase in roughness can greatly improve the contact area between the coating film and the substrate, and provide many effective anchoring points, which is conducive to improving the adhesion between the coating film and the substrate
.

The surface tension of the paint solution

When the paint is applied to the surface of the substrate, the old interface disappears and a new interface
is formed.
During the coating process, the degree of wetting of the coating to the substrate determines whether the coating film can form effective contact
with the substrate.

Due to the high surface tension of the water, wetting spreads on low-surface energy substrates are problematic, which also leads to some problems
.

Therefore, how coatings or waterborne resins can achieve low surface tension and excellent wetting spreadability is particularly important
.

When designing the formulation, the addition of substrate wetting agent should also be ensured to ensure that the coating can effectively and excellently wet the substrate
.

Coating thickness

Many experiments have shown that adhesion gradually decreases
as the coating film thickness increases.
Mainly because as the thickness of the coating film increases, its cohesion also increases, but the adhesion remains the same; When the cohesion increases greater than the adhesion, then the coating will have poor
adhesion.

From the perspective of the protective performance and fullness of the coating, the coating film should have a certain thickness, so a balance
should be made between these two aspects.

In addition, with the same total film thickness, the adhesion of multiple coatings is better
than that of one coating.

Drying process

During the drying process, ambient temperature and humidity have a certain influence
on adhesion.

The above is to test the influence of
temperature and humidity on the adhesion of a polyurethane water-based paint on glass.

In this experiment, the maintenance conditions of higher temperature and lower humidity have a certain promotion effect
on adhesion.

Paint viscosity

The viscosity of the paint solution will also affect the adhesion of the coating film to a certain extent, one is from the influence on the wetting spread, and the other is that the inappropriate viscosity will form stress to weaken the adhesion
.

Adhesion promoter

Certain adhesion promoters significantly improve the adhesion of paints, which are formed by a special chemical structure
.

According to different substrate types, different additives have different effects, common types such as epoxysilane
.

Face filler

A small amount of pigment filler can generally increase the adhesion of the coating, the principle is to reduce the cohesion energy
of the system.
Different pigments have different effects, for example, the improvement effect of scale-like pigments is more obvious
.

When the amount of pigment filler reaches a certain level, the adhesion will decrease
with the increase of the amount added.

solvent

Sometimes, solvents also have a significant effect on film adhesion, which is produced
by surface tension, drying and plasticizing effect on the paint film.
Most solvents have a relatively low surface tension, which is helpful for the wetting spread of water-based paint on the substrate and is conducive to improving the adhesion
of the coating film.

There are also solvents that evaporate slowly and can take weeks or even months to evaporate cleanly
.
When the solvent remains inside the paint film, the adhesion
of the coating film itself is weakened.

In addition, when the solvent gradually volatilizes after the paint film is cured, the paint film may shrink to a certain extent in this process, which will produce a certain degree of internal stress, resulting in poor adhesion
.

It can be seen from the above that the formation of coating adhesion and the influencing factors are multifaceted
.
When encountering adhesion problems, you should think
comprehensively about the problem.